The present invention, in some embodiments thereof, relates to self cleaning filtration systems and, more particularly, but not exclusively, to a nozzle assembly used for cleaning a filtration screen.
Over 80% of global trade volume is still transported by ship. Ballast water is required to compensate when there is an absence of cargo. A major environmental threat currently exists by the use of ballast water. Indigenous microorganisms, sediment, plants and animal life are drawn into the ballast water, shipped to another part of the world and then discharged into an alien environment. In response, the International Marine Organization has introduced the mandatory specification of automatic filters within ballast water treatment systems on all newly built and retrofitted vessels.
The cleaning of a filtration screen used for the filtration of water and sea water in particular is difficult. The lower the size of the holes in the filter (usually under 100 microns) the more difficult it is to remove the trapped suspended solids that have accumulated and imbedded on the filtration screen. Organic matter tends to stick to the filtration screen making removal especially difficult.
Some examples of attempts to increase the efficiency of cleaning a filtration screen include:
Nozzles that are fixed on the suction pipe collector at a preset distance from the filtration screen without an ability to move closer and further from the filtration screen.
Patent application WO 2006/008729 discloses “The nozzle 34 comprises a nozzle pipe 44 and nozzle cap 46 with an annular rim 47. The tubular basis 40 is closed by a cover 48. The nozzle pipe 44 is supported in the tubular basis 50 by an annular guide 50 and an opening in the cover 48 so as to form an axially movable telescope joint. The cylinder spring 42 urges the nozzle pipe 44 axially towards the inlet surface of the mesh 20.” Furthermore, “the telescope joint and the spring 42 are protected from the harmful particles by two tight sealing rings 52 and 54”.
Patent application EP 1785178 discloses a “nozzle 230 moves along guide 210, or at least along upper portion 212, while extending towards the inner part of fine filter 40.” Further, “During the backwash operation fluid flows through the final filtering chamber 30 toward the space defined within nozzle 240. Due to this flow the outer edge to 242 of nozzle 240 is pushed away from the fine filter 40 while the inner edge is pushed towards the fine filter. Because the area of the inner edge 244 is larger than the area of the outer edge 242 of nozzle 240 the overall effect of the fluid flow is to push nozzle 240 towards the inner face of fine filter 40.”